Storage batteries used for starting, lighting and ignition ("SLI batteries") in vehicles operated in hot arid climates are prone to failure more frequently than those in vehicles operated in more temperate climates. Evaporative water loss from the electrolyte solution caused by the hot arid environment is a major contributing factor to the increased failure rate. The reduction of such evaporative water loss is addressed by the floated insulating layer of closed cell particulate material disclosed in the parent application identified above. However, in the total design of a hot climate battery, additional features are desirable to ameliorate the effects of high temperatures and low humidity.
From analysis of failed batteries operated in a desert climate, it appears that primary failure modes attributable to the adverse climate include corrosion of the positive grids, positive plate growth resulting in short circuit contact against the underside of the negative intercell strap, and open circuits due to failure of the intercell welds.
Grid corrosion, principally the excessive creation of lead sulphate, can result from the exposure of the plates above the electrolyte solution, and from a highly concentrated electrolyte. Evaporative water loss both reduces and concentrates the solution. Consequently, it is generally recommended maintenance practice to add water as necessary to maintain the electrolyte level and specific gravity. It is also known that the temperature of the solution increases sulphation, and that lowering the specific gravity below the normal range tends to offset the effect of the temperature increase. In view of this knowledge, a battery specifically designed for such environment should preferably reduce evaporative water loss and have a lower specific gravity of electrolyte solution than a battery intended for temperate climates.
Positive plate growth arises from the shedding of active material from the plates, which is increased by high temperature and over-concentrated electrolyte. The exposed lead of the grid then leads to the formation of lead peroxide at irregular sites on the positive plates. This formation frequently occurs at the top corners of the positive plate near the surface of the electrolyte, as the growth creates stresses on the plate grids which cause them to buckle upward at the corners in the direction of growth. Buckling is accelerated by the design of the most prevalent modern grids, in which the vertical grid members have been replaced by thin grid wires directed radially inward toward the top center of the grid to provide better conduction paths to the grid lug. In addition, the number of horizontal grid wires in these "radial" grids is usually at least equal to, and in many instances greater than, the number of radial wires. This creates a grid composed of many small parallelograms with their angled shorter sides already directed away from the grid sides and which offers little resistance to upward buckling of the grid sides. Consequently, it is desirable to develop a grid design which resists upward buckling and to use additional means to reducing active material shedding in a hot climate battery.
Failure of the intercell weld is also a corrosion phenomena related to evaporative water loss. The weld is normally located near the surface of the electrolyte in a filled battery. Water loss increases specific gravity and lowers the electrolyte level, which exposes the weld and accelerates its corrosion, eventually leading to failure. Hence, it is desirable to increase electrolyte level and to reduce evaporative water loss in a hot climate battery.